Urgent international efforts to try and rescue more than 200 kidnapped Nigerian schoolgirls are facing "large information gaps," according to the UK experts who have started work to tackle the crisis.

The Nigerian government has been hit by stinging criticism, both at home and abroad, for being too slow to react since the 276 girls were snatched from their school in Chibok by Islamic extremists Boko Haram on April 15.

A range of international experts are now helping.

The UK advisory team has spent its first 24 hours since arriving in Abuja, the capital, in talks with senior officials including Nigerian president Goodluck Jonathan and the National Security Adviser Sambo Dasuki, to get to grips with the emergency.

A Foreign Office official: "The scale and complexity of the incident and the environment means there are large information gaps.

"The priority for the team in the first instance is establishing the facts such as the precise identities of those taken and what has actually happened to help Nigeria build a better picture."

They are still very much in the fact-finding rather than the tactical stage of dealing with the crisis and could not comment on reports that the search is closing in on a forest near the border with Cameroon and the girls have been divided into at least four groups which would make a rescue raid more difficult. Britain is working with the US counterparts and others to co-ordinate efforts.

China, France and Spain have also promised help.

Officials from the Department for International Development, Foreign and Commonwealth Office and Ministry of Defence (MoD) and and Metropolitan Police are now in Abuja.

The MoD has already linked up with the Nigerian armed forces, including the military which has operational control of the region where the kidnaps took place. There has a " very good meeting" with the Americans to begin figuring out the joint arrangements, areas of responsibility and what the next moves may be.

The Metropolitan Police, which includes the family liaison officers personally requested by Mr Jonathan in a meeting with Prime minister David Cameron, have met with Nigerian police to talk about how to tackle the huge demand for victim support.

Members of the UK team have also seen first-hand the pain and distress caused by the mass kidnapping in a meeting with a group which representing the Chibok families, the FCO spokesman said.

Mounting international outrage over the mass kidnapping has increased by an alarming report by Amnesty International which claims Nigerian commanders were warned that armed men were beginning to arrive near Chibok, but the military were unable to raise enough troops to respond.

Nigerian security forces had four hours of notice about the April 14 Chibok attack but did not react because of their fear of engaging the extremists, according to the Amnesty International report citing multiple interviews with credible sources.